1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine and an electrophotographic printer.
2. Description of the Related Art
Until now, an image forming apparatus using an electrophotographic method has been known in which an image carrier is brought into contact with a toner carrier to develop an electrostatic latent image (referred to as a contact development method) on the image carrier, transferring the image onto a recording media. In some contact-development image forming apparatus, the toner carrier is brought into contact with the image carrier during a development period, and the toner carrier is separated from the image carrier during a non-development period to prevent wearing-down of the image carrier and deforming of the toner carrier.
Japanese Patent Application Laid-Open No. 2003-323090 discusses a method for measuring the time during which the image carrier is in contact with the toner carrier in a case where the toner carrier is brought into contact with or separated from the image carrier as described above. In Japanese Patent Application Laid-Open No. 2003-323090, the control unit of the main body of image forming apparatus starts and stops the measurement of the contact time by transmitting a contact and a separation signal to a contact and a separation unit in a predetermined timing, and detecting the contact and the separation signal.
Since the measured contact time is an index of measuring a wearing degree due to mechanical stress to which the image carrier is subjected by the toner carrier, a replacement timing of the image carrier can be determined or various image forming conditions such as a development bias, charging bias, or transfer bias can be changed based on the contact time, for example.
The above conventional image forming apparatus, however, may not accurately measure the actual contact time between the image carrier and the toner carrier due to the following reason.
The contact and separation units for the toner carrier and the image carrier are formed of a large number of components. Dimensions of components of each unit and driving motors for driving the contact and separation units have variation among a plurality of image forming apparatus. Among the image forming apparatus using the electrophotographic method, there exists an image forming apparatus equipped with a process cartridge in which a plurality of members is integrated and made detachable from the main body of image forming apparatus so that consumables such as an image carrier and a toner can be readily replaced. Such a process cartridge is also formed of a large number of components. Dimensions of components at portions where the contact and separation unit of the main body of image forming apparatus act on the process cartridge have variation among the process cartridges.
In an environment (such as temperature and humidity) in which the image forming apparatus is used or in a case where a intermittent printing is repetitively performed in particular, not only the dimensions of components have variation but also members forming the contact and separation unit of the main body of image forming apparatus can cause settling and wearing-down. Similarly, in the portions where the contact and separation unit of the main body of image forming apparatus acts on the process cartridge the settling and wear may occur. For this reason, an actual time required when the toner carrier is brought into contact with or separated from the image carrier, is changed depending on situations where the image forming apparatus is used.
Because of the above reason, a contact time measured based on the contact and separation signal transmitted from the control unit of the main body of image forming apparatus is different from the time during which the toner carrier actually contacts the image carrier. Particularly, this becomes conspicuous as a printing speed of the image forming apparatus is increasing and lifetime of consumables (the process cartridge) becomes longer in recent years.